1. Field of the Invention
The present invention relates to electric plug-in contacts for plug-in connectors in electric DC wiring systems operated at a nominal voltage, in which electric arcing may occur, and to a semi-finished product for the production of such plugs as defined in the preamble of Claim 1. Especially, the invention relates to plug-in contacts intended for use in automobiles.
2. Description of Related Art
Today, passenger cars generally are equipped with electric wiring systems that operate at a nominal voltage of 14 V. Plug-in contacts in plug-in connectors intended for that use usually consist of a main body, made from copper, a copper-based alloy such as CuNiSi or stainless steel which provide the required electric conductivity and spring properties so ensure safe contact-making by the plug-in connector. In order to guarantee stable electric contact-making properties even in a corrosive atmosphere, it has been known to provide the main bodies of the plug-in contacts, or their semi-fished products, with a hard gold layer applied by galvanic deposition, or with a pure silver layer or a tin layer. Tin and tin alloy layers are frequently applied also using a fire-tinning process. Given the marginal conditions required heretofore (use in air at a nominal voltage of 14 V and a maximum ambient temperature of 150° Celsius—in which connection it must be considered that the temperature of the plug-in connector being heated by the current may further rise by 30° Celsius above ambient temperature) it has been possible in this way to achieve a sufficient degree of wear resistance when plugging-in and pulling off the male and female plug-in contacts.
It is planned by automobile manufacturers for the future to increase the voltage of passenger car wiring systems to 42 Volts. In direct-voltage electric wiring systems, operating at a nominal voltage of 42 V, plugging-in and pulling off plug-in connectors under load may give rise to electric arcing with the result that the plug-in contact or even the vehicle may catch fire. In addition, shocks occurring during operation of the vehicle may cause micro-interruptions between the plug-in contacts in the plug-in connectors, which likewise may give rise to electric arcs. Whether or not an electric arc actually strikes across a gap between two contact surfaces, due to vibration or under the effect of a plug being plugged in or pulled off, depends not only on the electric voltage available but also on whether a capacitive load or an inductive load is being separated and on the particular material making up the contact surfaces concerned. Generally, all materials have a specific minimum voltage, determined by physical conditions, which must be reached before charge carriers come off the contact surface to feed the electric arc. The element having the highest arc voltage is carbon with an arc voltage of 20 Volts; metals all have arc voltages of between 12 Volts and 16 Volts. In passenger car wiring systems, operating at a nominal voltage of 14 Volts, separation of a plug-in connector under load will not yet lead to a permanently firing arc as the load impedance in the previously closed circuit leads to a voltage drop so that the required arc-firing voltage will not be reached in the gap formed between the contact surfaces concerned when separating a plug-in connector. Consequently, the contact surfaces of today's passenger cars are not designed for loads at which electric arcing may occur.
The problem that in the case of 42 V automotive wiring systems electric arcing may occur when separating electric plug-in contacts, which may lead to serious damage and even to the vehicle catching fire and which are likely to jeopardize the safety of the passengers, is generally known in the art. Persons skilled in the art are also aware of the fact that the entire electric wiring system, and its components, will have to be redesigned to meet the requirements of a nominal voltage of 42 Volts and that considerable development effort will also have to be made for redesigning plug-in connectors and switchgear, see for example the paper by Thomas J. Schopf, “Electrical Contacts in the Automotive 42 VDC PowerNet”, Proceedings of the 21.sup.st International Conference on Electrical Contacts, 09 to 12.09.2002, Zurich, pp. 43 6o 55, especially p. 52. That paper suggests to investigate means for the quenching of arcs.
The paper by N. Ben Jamaa et al. “Short Arc Duration Laws and Distribution at Low Current (<1A) and Voltage (14-42 VDC)”, Proceedings of 20.sup.th International Conference on Electrical Contacts 19 to 23.06.2000, Stockholm, pp. 379 to 383, reports on investigations made regarding the arc duration on contact surfaces made from Ag, Au, Cu, Pd, Sn, Ni and steel, where palladium showed the shortest and tin showed the longest arc duration. However, palladium is very expensive which makes its use for plug-in connectors in the automotive field uneconomical. As mentioned before, the other materials are known as contact surfaces for plug-in connectors in 14V automotive power systems in modern vehicles, but are insufficiently qualified for use in 42V power systems.
Another problem is seen in the fact that electric arcs occurring in 42V automotive power systems may alter the contact surface and lead to a higher contact transfer resistance and, thus, to undesirable heating-up of the contact, under certain circumstances even to welding of plug-in contacts so that the latter can no longer be separated.